1. Field of the Invention
The present invention relates to a method and an apparatus for determining the local spherical and non-spherical (or aspherical) curvature of -rotating samples in multi-wafer epitaxial growth reactors as well as the related curvature variations across these samples. These samples typically are semiconductor wafers with epitaxial layers grown on their surface and the apparatus is intended to be applied during the epitaxial layer deposition.
2. Description of Related Art
During epitaxial growth of semiconductor wafers the growth processes have to be monitored in dependence of different parameters as, for example, the growing layer thickness, the temperature, the doping concentration, or the smoothness of the wafer surface. Among others, an accurate measurement of the curvature of the wafer is necessary because it correlates directly to the strain in the growing layers and indirectly to the composition of these layers. In particular during epitaxial thin-film growth on wafers in multi wafer reactors it is necessary to determine permanently the curvature of all wafers as exactly as possible in the shortest possible time. This is essential for facilitating high-yield epitaxial processes for the manufacture of semiconductor devices.
From U.S. Pat. No. 7,570,368 B2 a device is known for the measurement of the curvature of a surface, in which a single light beam and a two-dimensional detector are employed for determining the curvature of the surface, which is derived via the detection of the sample position as a function of time and from the position (x, y) of the light spot reflected from the surface to be measured and detected on the detector plane. U.S. Pat. No. 7,570,368 B2 assumes a uniform and strictly spherical bow at every point of the sample (spherical wafer bow approximation).
As it is sketched in FIG. 1, the simplest approximation for the bowing shape of a semiconductor wafer is to assume the same curvature C at every point (R,φ) with R<RS and φε[0, 360°), wherein RS is the radius of the circular wafer. The curvature C is measured as inverse bowing radius 1/rw. This approximation is named “spherical wafer bow approximation”, because the shape of the wafer bow z(R,φ) is identical to a segment of a sphere's surface with the corresponding sphere having a radius rw:C(R,φ)=const=1/rw. This approximation is also used in U.S. Pat. No. 5,912,738, in DE 103 61 792 B4, and in DE 10 2005 023 302 B4.
However, when determining the curvature of a double rotating semiconductor wafer (see FIG. 3) in a reactor during epitaxial growth, problems arise, due to the fact that real wafers are usually not uniformly bent, but the curvature may vary with the rotation angle, as depicted in FIG. 2a, thereby leading to unwanted fluctuations in the curvature measurements between a maximum value (along the direction of L2 in FIG. 2a) and a minimum value (along L1 in FIG. 2a). Moreover, if the epitaxial process is not adequately controlled, defects may be generated and relaxation processes may occur in the semiconductor wafers during fabrication, which according to their symmetry may further modify the rotational asymmetry (azimuthal aspheric bow) of the sample.
In such cases, specifically due to unavoidable wobble during satellite rotation ωsat, a solution according to U.S. Pat. No. 7,570,368 B2 cannot be used and also U.S. Pat. No. 5,912,738, U.S. Pat. No. 7,391,523 and DE 103 61 792 cannot be applied, because they can only determine the spherical curvature magnitude Csph.
Because in real semiconductor technology the wafer bow typically deviates from the ideal spherical bow, in European patent application 2,299,236 A1 the more realistic “azimuthal aspherical bow approximation” was used (named “azimuthal asymmetrical approximation” in EP 2,299,236 A1), according to which the local curvature of a wafer is described as an ideal spherical bow plus the superposition of an azimuthal (only φ-dependent) aspherical bow deviation.
Accordingly, European Patent Application 2 299 236 A1 is directed to a method and an apparatus for a real-time determination of the curvature of a rotating sample as well as the azimuthal asymmetry of the curvature. This is achieved by irradiating three parallel laser beams onto the sample surface and by detecting a first distance between the position of the reflected first light beam and the position of the reflected third light beam and a second distance between the position of the reflected second light beam and the position of the reflected third light beam. In this way, both curvature components Csph and the azimuthal aspherical curvature deviation ΔCasph can be measured in an epitaxial reactor with sufficiently large viewport size.
The separate measurement of those two curvature components (spherical curvature magnitude Csph and azimuthal aspherical curvature deviation ΔCasph) for a rotating sample results in a significant “noise” reduction in Csph measurements and in an access to strain-induced defect generation. As a drawback, said apparatus irradiating three laser beams can only be employed in epitaxial growth reactors having a medium-sized access window with a diameter between 5 mm and 20 mm or larger. This constraint follows from the fact that the distance between two laser beam ought not to be smaller that 0.5 mm and that the laser spot ought to have a minimum distance δ from the access window edge of 0.3 mm in order to avoid artefacts.
However, some epitaxial reactors, the so-called “close-coupled showerhead” reactors, enable only a very small access window, i.e. with a diameter between 1 mm and 5 mm, so that the size of the window is too small for irradiating three laser beams. FIG. 4 shows the projections of the incoming laser beams through an access window of an epitaxial reactor for an apparatus irradiating three beams (a) and for an apparatus according to an aspect of the present invention irradiating two beams (b), wherein w represents the edge of the access window through which the laser beams may access the epitaxial reactor, s indicates the center of the irradiated laser spots and δ is the minimum distance between the laser spot and the window edge in order to avoid artefacts.